Carbonaceous material can be reacted with steam at elevated temperatures to form syn gas, which is a combination of carbon monoxide and hydrogen. As disclosed in U.S. Pat. No. 6,863,878, if the initial reaction reaches a temperature greater than about 450° F. before the available oxygen is reacted, combustion occurs. This produces unwanted carbon dioxide, ash and slag. To avoid this, as disclosed in U.S. Pat. No. 6,863,878, the temperature must be maintained at 450° F. until after the available oxygen is reacted.